The invention relates generally to a color or monochrome electronic reprographic printing system, and more particularly concerns apparatus for optimizing the contact between paper or other copy media and a photoconductive surface.
In an electrophotographic printing machine, a photoconductive member (often a photoreceptor belt) is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is treated with toner particles and is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. Alternately, a plurality of images may be superimposed on the photoreceptor surface, and transferred simultaneously to the sheet. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
Surface irregularities in the paper may occur prior to use or during handling. Such irregularities are often caused by exposure to moisture, mishandling, duplexing, etc and create localized deformities in the copy paper. As a result, air gaps may form between the paper and the photoreceptor belt. Such gaps result in poor transfer of toner from the belt to the paper, which may, in turn, cause deletions or distortions in the printed copy. Flipping the paper over, or discarding the old paper and adding fresh paper offer possible solutions to this problem, but require the labor of frequent monitoring. The resulting rotation of paper stock is inherently expensive in paper costs, labor, and down time. Therefore, a means for reducing the need for operator involvement and reducing the amount of paper that is wasted is needed.
A device which applies a force against the back of a sheet and flattens it against the photoreceptor belt is one possible solution to the problem. U.S. Pat. No. 5,247,335, owned by Xerox Corp., describes a machine having such a device. The device described in the '335 patent employs a cam to move a wiper blade against the copy paper to facilitate engagement of the paper and photoreceptor belt.
Another Xerox Corporation patent, U.S. Pat. No. 5,227,852, describes another embodiment of a wiper blade which uses four flexible blade segments, each of which is deflected back away from the photoreceptor belt by solenoid actuated mechanisms. One or more of the solenoids are activated by the passage of a sheet, depending on the paper size being used. Since the blades of these machines are held in a deflected-back state both during standby and between each copy, the blade may tend to take on a permanent set over time, decreasing the force applied. This may result in the degradation in performance, over time, of the blades, and the need to replace the blades frequently.
There remains a need for a device that will provide enhanced contact between a copy sheet and a photoreceptor belt that is reliable and requires little maintenance.